In a typical cellular radio communication system (wireless communication system), an area is divided geographically into a number of cell sites, each defined by a radio frequency (RF) radiation pattern from a respective base transceiver station (BTS) antenna. The base station antennae in the cells are in turn coupled to a base station controller (BSC), which is then coupled to a telecommunications switch or gateway, such as a mobile switching center (MSC) and/or a packet data serving node (PDSN) for instance. The switch or gateway may then be coupled with a transport network, such as the PSTN or a packet-switched network (e.g., the Internet).
When a mobile station (such as a cellular telephone, pager, or appropriately equipped portable computer, for instance) is positioned in a cell, the mobile station (or MS) communicates via an RF air interface with the BTS antenna of the cell. Consequently, a communication path is established between the mobile station and the transport network, via the air interface, the BTS, the BSC and the switch or gateway.
With the explosive growth in demand for wireless communications, the level of call traffic in most cell sites has increased dramatically over recent years. To help manage the call traffic, most cells in a wireless network are usually further divided geographically into a number of sectors, each defined respectively by radiation patterns from directional antenna components of the respective BTS, or by respective BTS antennae. These sectors (which can be visualized ideally as pie pieces) can be referred to as “physical sectors,” since they are physical areas of a cell site. Therefore, at any given instant, a mobile station in a wireless network will typically be positioned in a given physical sector and will be able to communicate with the transport network via the BTS serving that physical sector. Because wireless communication with the BTS forms the basis of services offered by the operator (or owner) of the cellular radio communication system, physical sectors are also referred to as “wireless service sectors.” Both cells and sectors may be considered more generally as examples of wireless coverage zones, because they correspond to physical regions within which mobile stations may acquire RF links for access to the network.
As a mobile station moves between coverage zones, such as wireless service sectors of a wireless communication system, or when network conditions change or for other reasons, the mobile station may “hand off” from operating in one coverage zone to operating in another coverage zone. In a usual case, this handoff process is triggered by the mobile station monitoring the signal strength of various nearby available coverage zones, and the mobile station or the BSC (or other controlling network entity) determining when one or more threshold criteria are met. For instance, the mobile station may continuously monitor signal strength from various available coverage zones and notify the BSC when a given coverage zone has a signal strength that is sufficiently higher than the coverage zone in which the mobile station is currently operating. The BSC may then direct the mobile station to hand off to that other coverage zone. Without loss of generality with respect to the present invention, coverage zones will, for purposes of illustration, hereafter be considered to be wireless service sectors.
In some instances, the signal strengths of two or more sectors measured by a mobile station may each be within a particular threshold of a common reference level. For instance, the threshold could be 2 dB and reference level could be taken to be the strongest among the measured signals (in which case the strongest signal is, by definition, within any threshold of the reference level). Alternatively, a different reference level could be set by the BSC for each sector. Sectors determined to be within the threshold may then comprise a group, any or all of which could provide an air interface to the mobile station. Such a group is typically referred to as the mobile station's “active set,” and may be used to introduce a degree of efficiency into the handoff process, as well as to help the network construct a panoramic view of traffic load. As the mobile station continually monitors the signal strength of sectors, it may correspondingly update its active set as some sectors drop below and others rise above the threshold.
Some air interface communication protocols support sending the same transmission to a mobile station concurrently from two or more of the sectors in the mobile station's active set. Handoff under such a protocol may incorporate a certain seamlessness, as the mobile station can receive concurrent transmissions from multiple sectors as a matter of operation. Exemplary protocols of this type include one or another version of legacy CDMA, such as EIA/TIA/IS-2000 Rel. 0, A (hereafter “IS-2000”). Other air interface protocols specify communicating with a mobile station by transmitting at any one time from only one of the sectors in the mobile station's active set. An exemplary protocol of this type is EIA/TIA/IS-856 Rel. 0, A, or other version thereof (hereafter “IS-856”).
In some wireless communication systems or markets, both types of protocol may be implemented. Mobile stations operating in such systems may be capable of communication with either or both protocols, and may further be capable of handing off between them.